Laser Diffraction

Measuring the size of particles in suspended liquids or in dry powder format can be accomplished using particle size analysis based on the light scattering technique. Light refracted by a particle creates a scattering angle unique to its shape and size. The intensity of the scattering light is measured by an array of light detectors at different angles. The Mie Theory is the preferred method used to relate the measured scattering angle to the particle diameter. The Mie theory takes the complex refractive index of the particle into account when measuring the particle size. Nonetheless, in applications involving particles with high light absorption or with sizes significantly larger than the laser wavelength, the Fraunhofer Theory can be used to measure particle size. The Fraunhofer Theory is a simplified form of the Mie Theory; relating the scattering angle to the diameter of the particle and the light wavelength using the following equation:

where θ is the measured scattering angle, λ is the wavelength of the laser light and d is the diameter of the particle. The complex refractive index of the particle required for the Mie Theory is not needed for the Fraunhofer Theory. The light scattering technique has been readily applied to measure particle size and size distribution of particles with sizes ranging from 18nm to 2000µm.

Tornado Dry Particle System (left) and Universal Liquid Module (right) modules

Typical Experimental Results

Particle size of bottled water based on differential volume (top), surface area (middle) and
number (bottom).

Particle Size of Bottled Water Particles Based on Volume, Surface Area and Number Percentage

Particle size distributions based on differential volume (top left), surface area (top right) and number (bottom) for the tested coffee grounds made by Folgers, Starbucks and Dunn Brothers.

Particle Size of Coffee Grounds Based on Volume, Surface Area and Number Percentage

Applications

For more information please read our application notes:
Particle Sizing of Tap and Bottled Water, PDF
Particle Sizing of Coffee Grounds, PDF

Instruments: Beckman Coulter LS 13 320 Laser Diffraction Particle Size Analyzer with ULM and Sonication Control Unit connected

Key Specifications

Particle Sizing of Tap and Bottled Water

Water has great importance in human life. While water composes a majority of the Earth’s surface, only a small percentage is readily available for consumption. Safe, drinkable water is important for a healthy society. This does not mean water needs to be 100% free of impurities. Some impurities can be added naturally or artificially for health and safety reasons. Important minerals like magnesium and sodium ions occur naturally in the water supply. Trace amounts of chlorine can be added to control the growth of harmful microbes. Even bottled water may contain impurities, which are typically artificially added minerals or leeched material.

Measuring the size of the particles in water can be accomplished using particle size analysis based on laser diffraction technique. Light refracted by a particle creates a scattering angle unique to its shape and size. The intensity of the scattering light is measured by an array of light detectors at different angles. The Mie Theory is the preferred method used to relate the measured scattering angle to the particle diameter.

The LS 13 320 Laser Diffraction Particle Size Analyzer, manufactured by Beckman Coulter, is capable of measuring particles suspended in a liquid medium or in a dry powder form by using specially designed sample modules. The module capable of measuring particles in a liquid medium is the Universal Liquid Module (ULM). The ULM is capable of measuring size of particles from 0.040 µm to 2 mm. A wide range of liquids can be used in the ULM including ethanol, glycerol, mineral oils and jet fuel. The LS 13 320 is designed to be compliant with ISO 13320, the standard pertaining to particle size analysis through laser diffraction.

The data presented here is the size of particles found in tap and bottled water using the LS 13 320. Tables 1 and 2 show the particle size results for the tap and bottled water. Figure 1 shows the particle size distributions for the tap and bottled water based on volume, surface area and number percentages. Particle size analysis is useful in applications where knowing the presence of particles is important. Knowing the particle size distribution can be useful in determining contamination or saturation of liquid solutions.

Particle Sizing of Coffee Grounds

Coffee plays an important role in people’s daily life. To brew coffee, ground coffee beans are immersed in hot water to extract the flavor. Different sizes of coffee grounds are needed for various brewing methods. A French press, for example, uses coffee grounds that are very coarse, while the coffee grounds required for Turkish coffee are very fine. The grounds used for drip and Cappuccino makers fall in between those used in a French press and for Turkish coffee. Controlling the coffee ground particle size distribution is important in extracting the best flavor for the applied brewing method. Undesirable size distributions can lead to over extraction or under extraction of the coffee grounds, which affects the taste of the brew.

Measuring the size of the particles in coffee grounds can be accomplished using particle size analysis based on light scattering technique. Light refracted by a particle creates a scattering angle unique to its shape and size. The intensity of the scattering light is measured by an array of light detectors at different angles. The Mie Theory is the preferred method used to relate the measured scattering angle to the particle diameter. The Mie theory takes the complex refractive index of the particle into account when measuring the particle size. Nonetheless, in applications involving particles with high light absorption or with sizes significantly larger than the laser wavelength, the Fraunhofer Theory can be used to measure particle size. The Fraunhofer Theory is a simplified form of the Mie Theory; relating the scattering angle to the diameter of the particle and the light wavelength using the following equation:

Where θ is the measured scattering angle, λ is the wavelength of the laser light and d is the diameter of the particle. The complex refractive index of the particle required for the Mie Theory is not needed for the Fraunhofer Theory.

The LS 13 320 Laser Diffraction Particle Size Analyzer, manufactured by Beckman Coulter, is capable of measuring particles suspended in a liquid medium or in a dry powder form by using specially designed sample modules. The module capable of measuring dry powders is the Tornado Dry Powder System (DPS). The Tornado DPS is capable of measuring particle size of dry powders from 2mm to 0.4µm without a liquid medium. The LS 13 320 is designed to be compliant with ISO 13320, the standard pertaining to particle size analysis through laser diffraction.

The data presented here are three commercially available coffee brands, Folgers, Starbucks and Dunn Brothers. The Folgers and Starbucks coffee grounds came packaged from a local supermarket. The Dunn Bros. coffee grounds were freshly ground from a local franchise. All three brands are for a drip coffee maker. Using the LS 13 320 and Tornado DPS, the coffee grounds were sized. The results presented in Table 1 and Figure 1 show differences in size distribution among the prepackaged coffee grounds (Folgers and Starbucks) and the freshly ground coffee grounds (Dunn Bros.).